The invention pertains generally to scanning r for radio frequencies, and is more particularly directed to such receivers with the capability of communications with, and data base manipulation by, a remote processor.
A scanning receiver is generally known as a receiver which is adapted to receive information over a number of radio frequencies (RF) and bands. When placed in a scan mode, the receiver automatically steps between signals of different selected frequencies pausing at a signal long enough to allow the operator to determine if the channel information is of interest or not.
Early scanning receivers used crystals for tuning to the scanning frequencies, and the number of channels available for reception was limited by the number of crystals in a given receiver. A receiver typically contained either eight or sixteen crystals, and different crystals had to be installed to enable the reception of different frequencies. Particularly, whole sets of crystals had to be changed when it was desired to receive different bands, even when the same bands were desired but were assigned different frequencies in different geographical regions.
Many modern scanning receivers can generate the needed local oscillator frequencies across many bands. The local oscillator provides these frequencies with a digital frequency synthesizer which is controlled by digital frequency codes from a memory. Typically, the memory contains a small number of frequency codes which can be reprogrammed for operation on different frequencies. This limits channel capacity for the receiver, and, because of the need for frequent reprogramming, is only marginally better than switching crystal sets.
The channel capacity of scanning receivers has been limited not only because of the size, cost, and complexity of memory circuitry and associated addressing circuitry, but also because of technical limitations on scanning speed. A finite amount of time is required to lock on to each frequency in the actual scanning sequence, and then to detect activity on the current channel in order to determine whether or not to continue scanning. Large memories suggest widely separated frequencies which would slow scanning speeds to unacceptable levels.
Moreover, when a multiplicity of frequencies for a single band and a large number of bands are available for a scanning receiver, an operator can very easily lose track of which bands and frequencies are available for reception. Programming and changing bands becomes difficult to manage, and the reasons for storing particular channels in a band may be forgotten. Therefore, without sophisticated data management techniques, the capability of a large memory scanning receiver could not be used to its greatest advantage.
One significant disadvantage of scanning receivers with large memories is that receivers with memories usually scan the frequencies in the order in which they are input by the operator. This exacerbates the problem of increasing the scanning time where the frequencies may be separated by many MHz, reside in different bands, and even vary by type of detection. Even assuming the best case scenario, where the input frequencies are in the same general band and are of the same modulation type, they are still unordered. The slewing of a digital frequency synthesizer up and down in large increments slows the overall scan time because of the settling time. However, it is extremely inconvenient and unacceptable to expect an operator to insert every frequency in order when initially programming the scanning receiver. It is even more inconvenient and unacceptable to require this frequency ordering when reprogramming the receiver to insert another frequency in some band. This problem is significantly increased for scanning receivers which have coverage over a large number of bands and many frequencies within each band, i.e., those with larger memories.